This invention relates to titanium alloy wrought products. In particular, it relates to a method for improving the microstructure of wrought titanium alloys.
Titanium and titanium alloys are extremely valuable in uses where light weight and high strength-to-weight ratio are important. In order to fully take advantage of the strength and weight properties of titanium, it is often desirable to utilize hot or cold forming techniques to form complex shapes. Complex parts are parts having a shape of such complexity that they cannot be readily formed by standard casting, molding, forging, machining and welding techniques.
Sheet metal forming is typically used to form sheet stock of about 0.020 to 0.25 inch thickness. In superplastic forming, a die having a desired shape is used. A piece of stock of titanium alloy, such as a sheet of the alloy, is introduced into the die.
The part is normally heated in the die. In the die, the pressure on one side of the stock is reduced and the pressure on the other side of the stock is increased to establish a pressure differential of at least about 100 psi. The difference in pressure forces the stock to flow into the die and assume the desired shape conforming to the die.
This method of forming allows the fabrication of complex shaped, formed parts which can take full advantage of the high strength-to-weight ratios inherent in titanium and its alloys. To adequately superplastically form titanium and its alloys into useful parts, it is necessary for the stock to have a fine grain size (below about 10 microns) and, preferably, a microstructure which contains alpha and beta phases.
Lederich et al, U.S. Pat. No. 4,415,375 (Nov. 15, 1983) disclose a method for superplastically forming titanium and titanium alloys which comprises treating a stock piece of titanium or titanium alloy with hydrogen to form a transient alloy containing hydrogen, superplastically forming the hydrogen containing piece, and thereafter, removing the hydrogen from the formed piece.
Zwicker et al, U.S. Pat. No. 2,892,742 (June 30, 1959) disclose a process for hot working of titanium alloys which comprises incorporating about 0.05 to 1 weight percent of hydrogen into such alloys, hot working the hydrogen-containing alloys, and removing the hydrogen therefrom after the hot working has been completed.
Although Zwicker et al and Lederich et al have disclosed that hydrogen is beneficial as a transient alloying element for improving the hot workability and superplasticity of titanium and its alloys, pure titanium and many titanium alloys are embrittled at room temperature by the presence therein of only very small quantities of hydrogen. This embrittlement causes a lowered impact resistance. In order to obtain good mechanical properties at room temperature, it is necessary to remove the hydrogen therefrom after hot working or superplastic forming has been completed.
Further, the improved hot workability of titanium alloys containing hydrogen does not extend to alloys which are temporarily alloyed with hydrogen, then dehydrogenated under vacuum prior to hot forging. W. R. Kerr et al, "Hydrogen as an Alloying Element in Titanium (Hydrovac)", Titanium '80 Science and Technology, (1980) pp 2477-2486.
While it is desirable to improve the superplastic workability behavior of titanium alloys, it is less than desirable to have hydrogen present at the superplastic forming site.
Accordingly, it is an object of this invention to provide a method for improving the microstructure of titanium alloy materials, particularly wrought titanium alloy sheetstock materials.
Other objects and advantages of the present invention will be apparent to these skilled in the art from a reading of the following detailed description of the invention.